Increased sophistication of spacecraft and aircraft has resulted in improved weight-to-payload ratios. A part of this improvement is attributable to the use of lightweight composite materials for many structural portions of the vehicle. The use of composites for pressure vessels is well known. For example, U.S. Pat. No. 5,427,334, issued Jun. 27, 1995 in the name of Rauscher, Jr. describes a method for manufacturing a composite propellant vessel with a built-in vane.
One of the problems with the use of such composites on spacecraft or aircraft is that of the relatively great coefficient of thermal or temperature expansion (CTE) of composites, relative to other materials, such as steel. This problem is particularly acute, even in terrestrial applications, when the composite materials are used in or about cryogenic materials such as liquid hydrogen. The structure including the composite materials must initially be fabricated at temperatures at which humans can work, and may then be subject to extreme temperatures, such as -423.degree. F. (-253.degree. C.), when filled with cryogen. Extremely high temperatures may also be encountered under other conditions. The CTE of composite materials such as carbon-fiber reinforced polymer is large relative to other materials through the thickness, or in the direction a fastener would travel, such as steel, and its resistance to crushing forces is much lower.
When a steel screw or bolt is used with a nut to fasten together two structures, such as a hatch on a port of a pressure vessel, where both are made from composite material, the assembly is performed at room temperature, and if the nut is tightened to provide a force which is just below the force which would crush the composite material, the joint will loosen at cryogenic temperatures because the composite contracts more than the bolt. If the joint is exposed to a temperature significantly higher than room temperature, the joint will be crushed, because the composite material expands more than the bolt. Belleville washers are sometimes used to maintain tension in such structures, but tensioning the washer increases the tension in the bolt beyond that required for simply tightening the joint, thereby requiring a larger-diameter, and hence heavier, bolt. Also, Belleville washers must undesirably be carefully designed to the application.
Improved joints between composites are desired.